Sheep farmers are being warned to be highly vigilant for signs of a new killer virus as they approach the main lambing season.

The Schmallenburg Virus, causes birth defects in lambs – deformation of the head, neck and limbs – as well as blindness, and can also cause ewes to abort their lambs.

The disease can also strike cattle and goats.

It originates in Germany, where it has been responsible for the deaths of up to a quarter of lambs in almost 150 stricken flocks since it was identified last November.

Schmallenburg has yet to been made a notifiable disease, and development of a vaccine may be up to two years away.

So far, the virus has only struck in East Anglia and South East England, causing vets to believe it is spread by infected midges being blown across the North Sea and English Channel from the continent. At least 11 flocks in Norfolk, Suffolk, Essex, Kent and East Sussex have been hit. But, as the lambing season gets under way, the industry is expecting to hear of further cases – and there is concern that the virus could spread westward.

Ian Johnson, National Farmers' Union spokesman in the South West, said although all cases detected so far had been in eastern England, it was important that farmers should remain extra vigilant for signs of the disease and take all sensible precautions to prevent infection.

He said: "Cases on the continent have shown that the virus causes significant health problems for the animals with infection, resulting in still-born or deformed young.

"So any unusual symptoms, or abortions, should be reported to the local vet, or to the animal health office.

"If farmers are considering importing from the affected parts of continental Europe, we would strongly recommend that they discuss their plans with their vet first to reduce the risk of buying in the virus.

"Although this is still a relatively new virus, it is a developing situation and difficult to predict the scale of the problems."

Schmallenburg is the latest animal disease to spread to England from continental Europe. Four years ago Bluetongue Disease caused havoc among sheep flocks and cattle herds, having spread to Northern Europe from the Mediterranean, carried by midges. The farming industry itself took over the control campaign in a rolling programme of vaccination, which effectively checked its progress and then eliminated it.